This invention relates to compressor controls and more particularly to compressor controls in compressed gas systems having refrigerated dryers.
Refrigerant compressors are used in a variety of systems. One type of system that uses refrigerant compressors is a compressed gas system. Compressed gas systems typically provide high volumes of dry, pressurized air or other gases to operate various items or tools (while a multitude of gases can be used, this application typically refers to air as a matter of convenience). Conventional systems dry the air using heat exchangers first to cool the air and lower the dew point of the air, which causes water vapor to condense out of the air, and second to reheat the air and raise the outlet temperature of the air. This system provides a relatively dry air source. FIG. 1 shows a conventional refrigerated dryer 100 for a compressed gas system. Refrigerated dryer 100 includes both an air heat exchanger circuit 110 and a refrigerant heat exchanger circuit 120. Air heat exchanger circuit 110 includes an inlet 112, an air-to-air heat exchanger 114, a air-to-refrigerant heat exchanger or evaporator 116, a water separator 120a and an air outlet 118. Refrigerant heat exchanger circuit 120 includes evaporator 116, a compressor 122, a condenser 124, a throttling device 126, and a hot gas by-pass valve 128.
Notice that temperatures used below to describe the operation of dryer 100 are exemplary only. Many different air temperatures and saturation levels are possible. The temperatures and saturation levels of the final operating system depend on a large variety of factors including for example system design specifications and local environmental factors. The factors that determine actual temperatures are beyond the scope of this patent application and, in any event, are well known in the art.
In operation, dryer 100 receives a high temperature, saturated, pressurized air or gas stream at inlet 112. For example, the air or gas may be at 100 degrees (all degrees represented are degrees Fahrenheit) with a dew point of 100 degrees (i.e., 100% humidity), although any inlet temperature and dew point is possible. The air or gas stream passes through an inlet side of air-to-air heat exchanger 114. The air or gas stream cools down to, in this example, 70 degrees with a dew point of 70 degrees (i.e., still 100% humidity). However, because 100 degree air or gas can carry a larger volume of water vapor than 70 degree air or gas, some water vapor condenses. The condensed moisture precipitates out and collects in the separator 120a. The 70 degree air or gas then travels through the air side of evaporator 116 where the air or gas is further cooled to approximately 35 degrees with a dew point of 35 degrees (i.e., still at 100% humidity). Again, moisture condenses out of the air or gas stream and collects in the separator 120a. The 35 degree air or gas then travels through the outlet side of air-to-air heat exchanger 114. This reheats the air or gas stream to approximately 85 degrees with a pressure dew point of 35 degrees. The air or gas stream then exits the dryer 100 at air outlet 118. Because 85 degree air can hold significantly more moisture vapor than 35 degree air, dryer 100 provides a source of dry, unsaturated, pressurized air or gas at air outlet 118.
In refrigerant heat exchanger circuit 120, refrigerant enters the refrigerant side of evaporator 116 as a cool liquid. While passing through evaporator 116, the refrigerant heats up and is converted to a gas by the exchange of heat from the relatively hot air side to the relatively cool refrigerant side of evaporator 116. The low pressure gas travels to compressor 122 where the refrigerant is compressed into a high pressure gas. The refrigerant than passes through air or water cooled condenser 124 where the refrigerant is condensed to a cool, high pressure liquid. The cool, high pressure refrigerant passes through throttling device 126 (typically capillary tubes or the like) to reduce the pressure and boiling point of the refrigerant. The cool, low pressure, liquid refrigerant than enters the evaporator and evaporates as described above.
When air heat exchanger circuit 110 and refrigerant heat exchanger circuit 120 operate at or near full capacity, hot gas by-pass valve 128 has no particular function. However, as the demand on air heat exchanger circuit 110 decreases, refrigerant heat exchanger circuit 120 has excessive capacity that could cause the liquid condensate in dryer 100 to freeze. Thus, when used in this situation, hot gas by-pass valve 128 functions to prevent the liquid condensate in dryer 100 from freezing. In particular, the hot gas by-pass valve opens feeding hot, high pressure gas around the evaporator (i.e., by-passes) maintaining a constant pressure and temperature in the evaporator preventing any condensate from freezing. The particulars regarding the operation of hot gas by-pass valve 128 are well known in the art. Typically, a temperature sensor associated with the hot gas by-pass valve (not specifically shown in FIG. 1) monitors the refrigerant temperature at the outlet of evaporator 116. When the temperature at the outlet decreases below a predetermined threshold, the hot gas by-pass valve 128 opens feeding hot, high pressure gas around the evaporator maintaining a constant pressure and temperature in the evaporator preventing any condensate from freezing.
The capacity of compressor 122 depends, in large part, on the maximum required capacity or expected air flow (measured in standard cubic feet per minute) of air heat exchanger circuit 110. At full capacity (or air flow), compressor 122 operates at 100% capacity and the air temperature and dew point of the air stream is, for example, approximately as described above. The demand on the air system, however, is not always 100% of the designed capacity. Frequently, the demand on air heat exchanger circuit 110 is somewhat below full capacity. With less than 100% demand on air heat exchanger circuit 110, the refrigerant heat exchanger circuit 120 described above still operates at 100% capacity, thus wasting energy or electric power because compressor 122 does not need to operate at full capacity. Some systems, as described above, compensate using hot gas by-pass valve 128. Hot gas by-pass solves the problem of providing to much cooling through refrigerant heat exchanger circuit 120, but does not solve the problem that the compressor is operating at a higher than necessary capacity and consuming a larger amount of electrical power than necessary. Other systems cycle the compressor on and off when the system operates at less than 100% capacity. These systems reduce power consumption somewhat, but cause excessive on and off cycling of compressor 122, wide fluctuations in the dew point at air outlet 118, and introduce inefficiencies associated with the heat exchange of mass media. Thus, it would be beneficial to control operation of compressor 122 based on the demand of air heat exchanger circuit 110 to reduce the power consumed by compressor 122 and increase the overall power efficiency of dryer 100.
To attain the advantages of and in accordance with the purpose of the present invention, as embodied and broadly described herein, apparatus for controlling the operating speed of a variable speed compressor in a refrigerated air drying system having changing demands on an air supply, include a demand sensor capable of sensing changes in the demand on the air supply and generating a change in demand signal. A motor speed controller receives the generated change in demand signal and generates a motor speed signal. The motor speed controller sends the motor speed signal to a motor of the variable speed compressor to change the speed of the variable speed compressor.
Other embodiments of the present invention provide methods for controlling the operating speed of a variable speed compressor in a refrigerated air drying system having changing demands on an air supply. These methods include sensing a demand on the air supply. Determining an operating speed for a variable speed compressor based on the sensed air supply demand. Controlling a speed of the variable speed compressor based on the determined operating speed.
Still other embodiments of the present invention provide computer program products having computer readable code for processing data to control a speed of the variable speed compressor. The computer program product has a demand sensing module configured to sense changes in the demand of the air supply. A generating module is configured to generate a signal indicative of the sensed change in demand. A motor speed controlling module is configured to receive the signal indicative of the sensed change in demand and generate at least one motor speed signal. The motor speed controlling module is adapted to send the motor speed signal to the variable speed compressor.
The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.